The present disclosure relates to an input circuit to be suitably used for a semiconductor integrated circuit.
As a conventional semiconductor integrated circuit including a metal-oxide-semiconductor (MOS) transistor, a known input circuit is capable of inputting a signal larger in amplitude than a power supply voltage. For example, a known input circuit includes: an input terminal receiving an input signal; a power supply terminal for supplying a power supply voltage; an inverter; an N-channel MOS (NMOS) transistor having one end connected to the input terminal and another end connected to an input of the inverter and acting as a signal transfer transistor; and a P-channel MOS (PMOS) transistor having one end connected to the power supply terminal, another end connected to the input of the inverter, and a gate connected to an output of the inverter, and acting as a feedback transistor, wherein, between the power source terminal and the gate of the signal transfer transistor, a hold-clamp circuit is connected in order to cope with the power supply voltage falling and reduce a propagation delay of the input signal. The hold-clamp circuit includes: a diode-connected NMOS transistor; a high-resistive element; and a diode-connected PMOS transistor all of which are connected in parallel. (See Japanese Unexamined Patent Publication No. H11-243330.)
In the above conventional technique, the gate voltage of the signal transfer transistor is not always held constant to the power supply voltage by the high-resistive element. Moreover, when the input signal rises, a voltage clamping operation of the diode-connected NMOS transistor stops at a certain point the rise of the voltage of the gate of the signal transfer transistor. When the input signal falls, the voltage clamping operation of the diode-connected PMOS transistor stops at a certain point the fall of the voltage of the gate of the signal transfer transistor. However, when the input signal falls, the gate voltage of the signal transfer transistor inevitably falls, causing an inevitable propagation delay of the input signal.